1. Field of the Invention
The present invention is related to a photothermographic material which is used advantageously in the fields of films for medical diagnosis and films for photoengraving.
2. Description of the Related Art
Reduction in the amounts of waste processing solutions has been strongly desired in recent years in the field of medical imaging from the viewpoints of environmental protection and space saving. Under such circumstances, technologies on photothermographic image-recording materials as films for medical diagnosis and photoengraving which can be exposed to light efficiently with a laser image setter or a laser imager, and can form a clear black image having high resolution and sharpness have been demanded. With these photothermographic materials, it is possible to supply to customers a heat development treatment system which has eliminated the necessity of using solvent processing chemicals, and is simpler and does not impair the environment.
The similar requirements also exist in the field of general image forming materials. However, the image for medical use is required to have a high image quality excellent in sharpness and graininess, because fine details of the image are required. In addition, the medical image is characterized by preferably exhibiting a blue black image tone from the viewpoint of ease of medical diagnosis. Currently, various hard copy systems utilizing pigments or dyes such as inkjet printers and apparatuses for electrophotography are prevailing as general image forming systems. However, there is no system which is satisfactory as a medical image-output system.
On the other hand, thermal image forming systems utilizing organic silver salts are described in, for example, U.S. Pat. Nos. 3,152,904 and 3,457,075, and in “Thermally Processed Silver systems” (Imaging Processes and Materials), Neblette, 8th edition, edited by J. Sturge, V. Warlworth, and A. Shepp, Chapter 9, page 279 in 1989. In particular, photothermographic materials generally have an image-forming layer in which a catalytically active amount of photocatalyst (for example, a silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt) and, optionally, a color tone adjusting agent for controlling the tone of a developed silver image are dispersed in a binder matrix. When the photothermographic material is imagewise exposed to light and then heated to a high temperature (for example, 80° C. or higher), a redox reaction between the reducible silver salt (functioning as an oxidizer) and the reducing agent occurs to form a black-toned silver image. The redox reaction is promoted by the catalytic activity of a latent image of silver halide formed by exposure. Accordingly, the black-toned silver image is formed in an exposed region.
Such photothermographic materials have been already known. However, in many of these recording materials, the image-forming layer is formed using an organic solvent such as toluene, methyl ethyl ketone, or methanol. It is not advantageous to use an organic solvent since the organic solvent may cause harmful effects on human during the production process of the recording materials, and since it is costly to collect the solvent and to conduct other related processes.
In order to solve such problems, a method has been proposed in which a water-based coating liquid is used for forming an image-forming layer (hereinafter sometimes referred to as “water-based photosensitive layer.” For example, techniques of using gelatin as a binder are disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 49-52626 and 53-116144. Further, a technique of using polyvinyl alcohol as a binder is disclosed in JP-A No. 50-151138.
However, these techniques are not practically satisfactory since the fogging level is significant and the tone of a formed image is not good. On the other hand, a technique of using polymer latex binder and water-based medium for forming an image-forming layer is disclosed in JP-A Nos. 10-10669 and 10-62899.
Photographic photosensitive materials usually contain a dye for, for example, prevention of halation, prevention of irradiation, filtering function or adjustment of image color tone. There are also similar requirements for photothermographic materials. However, unique functions that are not found in conventional wet-developing photosensitive materials are also required for the photothermographic materials. That is, all raw materials needed for image formation should be previously contained in the coated film, and these raw materials and reaction products thereof remain therein even after thermal development.
Generally, dyes have not only absorption in a required wavelength region but also side absorption at wavelengths longer and shorter than the range, and therefore, for example, an infrared absorption dye for infrared exposure has absorption in the visible region, resulting in a visually observable color. For this reason, decoloration of a dye used in a photothermographic material by a chemical reaction occurring due to heating during thermal development has been attempted. However, introduction of such a decoloration mechanism results in problems such as increase in turbidity and film thickness, and due to adverse effects on the photographic characteristics of the resulting image, introduction thereof into the image-forming layer or neighboring layers is difficult. In addition, slowness of a decoloration reaction makes it difficult to rapidly perform decoloration.
Alternatively, use of dyes with a narrower absorption spectrum that have smaller side absorption, without a decoloration mechanism, has also been attempted (e.g., U.S. Pat. No. 6,830,879 and JP-A No. 2003-84395). Characteristically, these dyes exhibit a narrow absorption spectrum by forming an aggregate and effectively have a maximum absorption wavelength in the required wavelength region, and therefore are effective even with a small amount.
However, there are many requirements for such dyes because photothermographic materials are used in various environments, and these dyes still do not sufficiently satisfy these requirements. In particular, under the circumstance inherent to the photothermographic materials, that the dye remains and is stored in the film even after image formation, there are new unexpected problems that have not been encountered in the past, and thus these exists a need for improvement and new methods of solving such problems.
Accordingly, there exists a need for a photothermnographic material that has good sharpness and good image storability and provides a preferable image color tone.